1. Field of the Invention
The present invention relates to a light-emitting apparatus, and more particularly to a light-emitting apparatus that irradiates a phosphor with primary light emitted from a primary light source and that thereby produces secondary light having longer wavelengths than the primary light.
2. Description of the Prior Art
GaN-based semiconductors are direct-transition semiconductors with band gaps ranging from 0.9 eV or 1.8 eV to 6.2 eV, and thus they make it possible to realize light-emitting devices that emit light in a wide band ranging from visible to ultraviolet regions. For this reason, in recent years, GaN-based semiconductors have been receiving much attention and have been actively developed. The reason that the lower limit of the band gap is given as 0.9 eV or 1.8 eV above is that the band gap of InN has not yet been definitely determined between two theories that support 0.9 eV and 1.8 eV respectively.
As is widely practiced, such a GaN-based light-emitting device is used as an excitation light source to produce white light by mixing together phosphorescence of different colors emitted from red, green, and blue phosphors irradiated with light emitted from the GaN-based light-emitting device.
It has also been proposed to use a GaN-based light-emitting device in a full-color image display apparatus (Japanese Patent Application Laid-Open No. H8-63119). In this full-color image display apparatus, either phosphors each emitting phosphorescence of one of three primary colors, namely red, green, and blue, are excited by a GaN-based light-emitting diode array arranged on a substrate, or phosphors each emitting red or green phosphorescence are excited by a GaN-based light-emitting diode array and blue light is emitted directly from GaN-based light-emitting diodes.
On the other hand, next-generation light-emitting apparatuses are desired to offer high brightness combined with low power consumption. The brightness and power consumption of a light-emitting apparatus depend on the output power and quantum efficiency of its excitation light source and on the quantum efficiency of its phosphors. Thus, the phosphors are desired to have as high quantum efficiency as possible. Moreover, the resolution of a full-color display apparatus depends on the size of its pixels, and therefore, in a case where a phosphorescent surface is formed by applying a phosphorescent paint on a surface, it is necessary to reduce the size of the crystal particles of the phosphorescent material to suit the size of the pixels.
However, conventionally available phosphors have quantum efficiency of 10% or lower, and therefore, to obtain higher brightness, it is necessary to increase the light output power of the excitation light source. Inconveniently, this increases power consumption and shortens the lifetime of the excitation light source. For this reason, it has to date been difficult to realize a light-emitting apparatus that uses a GaN-based light-emitting device as an excitation light source and that offers high brightness combined with low power consumption and a long lifetime.
Recently, it has been observed that reducing the size of a crystal down to the exciton Bohr radius (hereinafter, such a crystal will be referred to as a “nano-crystal”) causes, owing to the quantum size effect, trapping of excitons and an increase in the band gap (J. Chem. Phys., Vol. 80, No. 9, p. 1984). It has been reported that some semiconductors of such size exhibit photoluminescence with high quantum efficiency (Phys. Rev. Lett., Vol. 72, No. 3, p. 416, 1994; MRSbulletin Vol. 23, No. 2, p. 18, 1998; and U.S. Pat. No. 5,455,489).
Now, this effect will be described in the case of Mn-doped ZnS (ZnS:Mn), which is taken up as an example here for easy comparison because its light emission wavelength does not vary under the quantum size effect. Table 1 shows the results of comparison of the brightness of light emission obtained when ZnS:Mn nano-crystals having their surface treated with methacrylic acid and bulk ZnS:Mn particles having a particle size of 1 μm or greater were excited with the same ultraviolet lamp. Table 1 shows that the ZnS:Mn nano-crystals offer brightness close to five times the brightness offered by the bulk ZnS:Mn particles.
TABLE 1Nano-crystalsBulk ParticlesBrightness69 cd/m214.2 cd/m2
How this high quantum efficiency physically relates to the quantum size effect has not yet been definitely explained, but the possible factors that are considered to be involved include an increase in the intensity of excitons which results from the formation of electron-hole pairs, a decrease in the state density that does not contribute to light emission which results from the quantization of energy levels, a variation in the crystal field near the center of light emission which results from distortion of the crystal lattice, and surface treatment of the crystals. Which of these factors contributes effectively to light emission efficiency is not clear, but light emission efficiency has been reported to increase in crystals having sizes smaller than the exciton Bohr radius, which will be described below.
Here, the exciton Bohr radius indicates the extent of the probability of existence of excitons, and is given by 4π ∈0 h2/me2 (where “∈0” represents the low-frequency dielectric constant of the material, “h” represents the Planck constant, “m” represents the reduced mass obtained from the effective masses of an electron and a hole, and “e” represents the electric charge of an electron). For example, the exciton Bohr radius of ZnS is about 2 nm, and that of GaN is about 3 nm.
A most typical example of the quantum size effect is an increase in the band gap. FIG. 1 shows the dependence of the band gap on the crystal size as calculated on the basis of the theory by L. E. Brus et al. The intrinsic band gap of ZnS is about 3.5 eV, and therefore the quantum size effect is expected to increase in the range where the diameter is smaller than about 8 nm. This diameter is that of a crystal having a radius equal to twice the exciton Bohr radius.
Accordingly, by using a phosphor formed of crystals having a size equal to or smaller than twice the exciton Bohr radius, it is possible to exploit the contribution of the quantum size effect to light emission. That is, by varying the size of nano-crystals, it is possible to obtain different phosphorescence wavelengths. As nano-crystal materials having high quantum efficiency other than ZnS, there are actively being studied II-VI group materials such as CdSe.
Moreover, as shown in FIG. 2, a CdSe nano-crystal capped with ZnS has a quantum well structure, in which electron-hole pairs are strongly trapped inside the nano-crystal and thus undergo recombination. This material offers light emission efficiency higher by an order or more of magnitude than that of an uncapped CdSe nano-crystal, and offers quantum efficiency of about 50%.
A display apparatus and an illumination apparatus using a II-VI group nano-crystal material has been proposed (Japanese Patent Application Laid-Open No. H11-340516).
However, II-VI group materials have the following disadvantages. The result shown in Table 1 is that obtained when the nano-crystals were subjected to surface treatment using methacrylic acid. However, in crystals that are not subjected to surface treatment, excited electrons are captured by the dangling bond of ions existing on the surface and undergo non-radiation recombination. This greatly reduces light emission intensity. For example, as shown in Table 2, in ZnS:Mn nano-crystals of which the surface is not treated with methacrylic acid, the dangling bond on the surface of the crystals is not effectively terminated, with the result that the light emission intensity of these nano-crystals is far lower than that of the sample of which the surface is treated. Thus, II-VI group nano-crystals require a special process to stabilize its surface.
TABLE 2Surface TreatmentMethacrylic AcidNoBrightness69 cd/m29.4 cd/m2
Moreover, II-VI group materials contain toxic substances such as Cd and Se, and therefore using II-VI group materials in light-emitting apparatuses and image display apparatuses is a matter of great concern from the environmental perspective.